A known example of such an optical fiber comprises cladding made of silica SiO.sub.2, and cores containing a quantity of germanium oxide GeO.sub.2. Such a dopant imparts a refractive index to the cores that is different from the refractive index of the cladding. In that way, each core acts as a waveguide for light so that the information transmission capacity of the multi-core optical fiber is multiplied by the number of cores it includes. The cores are generally disposed symmetrically about an axial generator line of the multi-core optical fiber.
Connecting multi-core optical fibers is more fiddly than connecting single-core fibers. It is necessary to locate the cores individually in a connection plane that is perpendicular to the optical fiber so as to prevent any risk of erroneous interconnection. The symmetrical disposition of the cores is not sufficient to locate them individually.
One solution consists in marking the multi-core optical fiber during its manufacture. A coating of plastics material is disposed around the optical fiber, and then marked with a colored groove using an ink. That type of marking requires high accuracy, given the diameter of the multi-core optical fiber, and runs the risk of degrading the mechanical strength of the optical fiber. The manufacturing method must also include a step dedicated to locating the cores.
In a solution known from document FR-2 736 441 A1, the multi-core optical fiber is surrounded by a ring whose outer edges reproduce, in geometrically similar manner, the edges of the optical fiber. A colored mark is made on one of the outer edges of the ring for location purposes. It should be observed that that solution is not applicable to multi-core optical fibers of circular section. Once again, a step of the manufacturing method must be dedicated to locating the cores.
Another solution consists in giving a reference role to one of the cores of the optical fiber, by giving it an optogeometric property that is different from that of the other cores.
An example of such a solution is given by a multi-core optical fiber in which dopant is absent from one of the cores. Observing a connection plane under a microscope enables all the cores to be seen apart from the non-doped core, since it is the dopant which gives rise to contrast between the cores and the cladding which results from the difference between the refractive indices. The symmetrical disposition of the cores in the connection plane is thus broken by the non-doped core which provides a local reference point. The non-doped core is, however, no longer used for transmitting information and reduces the capacity of the optical fiber.
In another solution, the dopant for one of the cores is distributed over a section that is much smaller or much larger than the section of the other cores of the optical fiber, in order to provide a local reference point. However the difference in diameter of the sections is such that the core of much smaller or much larger section is not used for transmitting information, resulting once again in the capacity of the multi-core optical fiber being reduced.